1. Field of the Invention
This invention relates to a method and apparatus for both implementing a braze melt, and detecting when the braze melt occurs. Generally speaking, the invention comprises a radiant brazing heater having an electrical resistance coil assembly which serves the dual functions of providing a brazing heat onto a brazing material while simultaneously acting as an eddy current probe. The invention is particularly useful in brazing Inconel reinforcing sleeves within the heat exchange tubes of nuclear steam generators, where the utilization of the coil as an eddy current probe allows the operator to accurately determine when the brazing material surrounding the reinforcing sleeves melts.
2. Description of the Prior Art
Devices and processes for applying a brazing heat to the inside surfaces of conduits are known in the prior art. Such devices and processes are used to braze reinforcing sleeves within the heat exchange tubes of nuclear steam generators. In such prior art processes, a sleeve which is circumscribed at each end by a ring of brazing alloy is inserted within a tube in need of repair and slidably positioned across the section of the tube where the walls have been damaged due to corrosion or denting. The ends of the tube are then usually hydraulically expanded so that they snugly engage the inner walls of the tube being repaired. To complete the joint, a heater assembly which may be formed from an electrical resistance wire coiled around a mandrel is slid up into the sleeve and actuated so that radiant heat melts the rings of brazing alloy disposed between the outer wall of the sleeve and the inner wall of the tube, thereby forming water-tight braze joints between the ends of the sleeve and the tube. Such prior art sleeving operations are frequently performed in the longitudinal sections of the heat exchange tubes which extend through the tubesheet of the nuclear steam generator due to the tendency of these sections of the tubes to corrode, dent and crack.
One of the problems associated with the implementation of such brazing processes is the application of the proper amount of radiant heat to the brazing material. If the electrical resistance coil is actuated for an insufficient amount of time, the brazing material might not uniformly melt and flow in the annular space between the reinforcing sleeve and the tube being repaired, thereby resulting in a poor-quality braze joint which would not create the desired water-tight seal between the sleeve and the tube. On the other hand, if an excessive amount of heat is applied to this brazing material, the brazing material may be liquified to the point where it is too thinly spread between the outer walls of the sleeve and the inner walls of the tube, which again could result in a poor-quality braze joint.
One attempted solution to this problem involves the actuation of the electrical resistance coil for an "average" amount of time at a given power level, wherein the "average" is an empirically determined quantity based upon a statistical analysis of a series of experimental brazings. However, because of wide variations in the heat-sink properties in any given number of sleeve/tube combinations, the use of an empirically derived "average" amount of heating time has not resulted in a completely satisfactory solution to the problem. It is believed that a principal cause of these wide heat-sink variations is the amount of dark-colored oxides on the outside walls of the tubes being brazed. The presence of a layer of such dark-colored oxides on the outside walls of the tubes can greatly increase the radiant heat losses out of the sleeve/tube combination due to the phenomenon of black-body radiation. Other causes of heat-sink variations may include the amount and type of corrosion between the tube and the tubesheet (some oxides are fairly good heat conductors), as well as the amount of water surrounding the tube being brazed. These variations make it difficult to consistently apply the proper amount of radiant heat to the inside of the sleeve/tube combination so that the heater assembly melts the ring of brazing alloy to the desired extent to successfully braze the outer walls of the reinforcing sleeve to the inner walls of the surrounding heat exchange tube.
Clearly, there is a need for some means for correctly and reliably determining the amount of time that the heating element of a radiant brazing heater should be actuated in order to consummate a strong, water-tight braze joint in the space between the outer walls of the reinforcing sleeve and the inner walls of the surrounding heat exchange tube. Ideally, such means should be capable of precisely determining the proper length of time that the radiant brazing heater should be actuated, regardless of the particular heat-sink properties of the sleeve/tube combination being brazed. Finally, such means should be conveniently usable in conjunction with radiant braze heaters, rugged in construction, and capable of withstanding the brazing heat generated by the coil.